1. Field of the Invention
This invention relates to investigating earth formations traversed by a borehole. More particularly, the present invention is directed to an apparatus and a method for determining element concentration values and for further characterizing the attributes of the formations surrounding a borehole.
2. Related Application
The present application is related to the copending Assignee's patent application filed Dec. 13, 1989, Ser. No. 07/450,355, in the name of Don McKeon et al., for a "Logging Apparatus and Method for Determining Concentrations of Subsurface Formation Elements", wherein the apparatus comprises a single neutron source in the form of an electronic high energy neutron source.
3. The Related Art
The capital cost of drilling and evaluating a deep well, for example an oil or natural gas well, is extremely high, and for this reason considerable expense is incurred during those time intervals when drilling or production steps must be interrupted to evaluate the formation. With known analysis techniques, the concentration of some elements might be derivable from logging of the formation, but the concentrations of other elements would require the taking of core samples for analysis.
The existence and quantity of an element in a formation can be determined, as described in U.S. Pat. No. 3,665,195, by irradiating the formation with neutrons and detecting the induced gamma ray activity from the element of interest. After determining the thermal neutron capture cross section of the formation, the product of the gamma ray emission and the thermal neutron capture cross section is obtained as a quantitative indication of the element's abundance in the formation.
Here after is given a simplified view of the thermal neutron capture process. Neutrons are created and propagate into the formation. Some of the neutrons are absorbed, but the majority slow down until they reach thermal energies. At thermal energies, the neutrons diffuse until they are captured by one of the nuclei of the formation. For a particular neutron, its capture will depend on the number of nuclei it "sees", weighted by the microscopic capture cross section (probability) of each nucleus (the total in a homogeneous formation is proportional to the formation capture cross section SIGMA.sub.form). Thus, the greater the number of nuclei of a particular element, the greater the number of neutrons that will be captured by that element. In other words, the number of capture gamma rays produced is, for a particular element, proportional to the number of nuclei per volume unit. Once the neutrons are captured, they will produce a spectrum of prompt capture gamma rays specific for each element. These gamma rays are transported to the detector, some are degraded in energy, and others are lost. Those that are detected by the detector are used for the spectral measurement. This detected spectrum is decomposed to obtain the fractional contributions or yields, Y.sub.i, of each element in the total spectrum. Relative values for two Y.sub.i will be proportional to the relative atomic abundance of the elements in the formation (with the ratio weighted by many nuclear parameters: gamma-ray multiplicity, gamma-ray transmission probabilities, neutron capture cross sections, etc). Once good relative yield measurements are provided, it is only necessary to determine the proper absolute normalization to transform these relative measurements into elemental concentrations.
In the article "The Aluminum Activation Log" by H. D. Scott and M. P. Smith, presented at the SPWLA Fourteenth Annual Logging Symposium, Lafayette, Louisiana, May 6-9, 1973, there is described a method for measuring the aluminum content of the formation, in order to estimate the formation shale fraction. A californium-252 source of neutrons is used in conjunction with a measurement of the formation thermal neutron capture cross section to produce a continuous activation log of a borehole.
In the neutron activation process, an atomic nucleus absorbs a neutron, creating an unstable isotope which decays, after some delay, usually by beta decay, with associated gamma rays of characteristic energies. In aluminum activation, the natural isotope 27.sub.Al absorbs thermal neutrons and produces the unstable isotope 28.sub.Al, which beta decays with a half-life of 2.24 minutes, emitting a 1779 keV gamma ray. This sequence is summarized below: ##STR1##
As a general definition, here "capture" refers to the prompt emission of gamma rays, while "activation" here refers to the delayed emission of gamma rays.
U.S. Pat. No. 4,464,569 discloses a method for determining basic formation component volume fractions, including a spectroscopic analysis of capture gamma ray spectra obtained from a neutron spectroscope logging tool. The relative sensitivities of the logging tool to the specific minerals or to the chemical elements in the formation are determined either from core analysis or from tests run in known formations. The spectroscopic elemental yields and the relative sensitivities are then used together to determine the volume fractions of the basic formation components such as limestone, sandstone, porosity, salinity, dolomite, anhydrite, etc.
Nevertheless, the method described in the '569 patent does not require, and the patent does not disclose, a straight forward way for determining elemental concentrations, especially through the use of commonly available logging tools or modifications thereof. This known method takes appropriate combinations of measured yields, normalizes core data or laboratory measurements to obtain calibrated relative sensitivities and makes use of the constraint that the sum of all volume fractions is unity. Values of the volume fractions can then be found by solving the appropriate set of equations for the formation component volume fractions.
U.S. Pat. No. 4,810,876 contemplates a logging apparatus and processing methods for determining elemental concentrations, in order to assess the mineralogy of a formation, based on an indirect approach that in part relies upon certain unique assumptions.
The article entitled "Geochemical Logging with Spectrometry Tools" by R. Hertzog et al., presented at the 62nd Annual Technical Conference and Exhibition of the SPE, held in Dallas, Texas on Sept. 27-30, 1987, paper SPE #16792, discloses a Geochemical Logging Tool, known as the GLT tool (mark of Schlumberger Technology Corporation), designed to measure natural, activation, and neutron capture gamma rays. The GLT tool produces logs of the most abundant elements and direct measurements of Al concentrations are provided. The GLT tool comprises a tool string including successively from top to bottom: (i) a natural gamma ray tool, known as the NGS tool (mark of Schlumberger Technology Corporation) and depicted in U.S. Pat. No. 3,976,878; (ii) a source of low energy neutrons preferably Californium-252; (iii) an activation tool, known as AACT tool, adapted for measuring the delayed gamma rays resulting from the activation of aluminum atoms by the neutrons emitted by the californium source; and (iv) a gamma spectrometer tool, known as the GST tool (mark of Schlumberger Technology Corp.) and being such as depicted in U.S. Pat. Nos. 4,317,993 or 4,327,290; the GST tool is designed to detect prompt gamma rays resulting from the capture of neutrons emitted by another source, i.e. a high energy (14 Mev) neutron generator provided in the string. The whole GLT tool involves three separate modes of gamma-ray spectroscopy to make a comprehensive elemental analysis of the formation. The first measurement is performed by the NGS tool which passes by the formation before any neutron source can induce radioactivity in order to derive the concentrations of K, Th, and U in the formation. The second measurement is performed by the AACT tool; the AACT tool, the NGS tool above it, and the .sup.252 Cf neutron source between them, allow a measurement of activation gamma rays to be used to derive formation aluminum concentration. The third measurement is performed by the GST tool to derive a spectrum of capture gamma rays from a plurality of elements in the formation, such as Si, Ca, Fe, S, Ti, K, and Gd. The GST tool uses a high energy (14 Mev) pulsed neutron generator to induce these capture reactions.
Although the above mentioned known GLT tool affects significant advantages over earlier tools, it is desirable to provide still further improvements.
Due to the relatively large number of devices composing the GLT string, the GLT turns out to be critically long. This drawback, detrimental by itself, also indirectly prevents any improvements of the measurements by adding detectors and/or electronic data processing devices.
Moreover, the logging speed is relatively slow due to the fact that the "capture" measurements cannot be continuously carried out, but only during the time the pulsed neutron generator is off. Furthermore, the measurements of capture gamma rays are carried out in a time window, after the source has been turned off, when the counts have already drastically decreased; this substantially reduces the precision of the measurements.
Additionally, the presence of the high energy neutron generator prevents placing, at the bottom of the string, an additional tool designed to carry out density measurements, because of the interference which might occur between the gamma rays emitted by the density tool and the gamma rays resulting from the fast neutron reaction of the atoms of interest (S.sub.i and O.sub.2) of earth formations.
Finally, since two different sources, i.e. the radioactive californium source and the high energy neutron generator, are respectively used for the aluminum activation measurements and the "capture" measurements, an environmental correction for aluminum is required. Such correction, needed for taking into account the porosity and the absorption properties of the formation and of the borehole, is not fully satisfactory due to its relatively empiric nature.
According to the above, there is a need for a logging tool for measuring natural, activation and neutron capture gamma rays, which do not show the drawbacks hereabove mentioned.